[unreadable] Dedicated high-resolution small animal imaging systems have emerged as important tools for cancer research. These systems permit noninvasive screening of mutations or pathologies, monitor disease progression and response to therapy. MicroPET has demonstrated a tremendous potential in the study of small animal models, allowing the measurement of biological parameters and permitting in-vivo imaging of transgene expression. [unreadable] [unreadable] Current trends are towards combining PET with CT so that the functional information from PET can be co-related to the anatomic information provided by CT. However, the current X-ray detectors, which provide excellent spatial resolution, but have low sensitivities, limit the use of this powerful combined modality and poor signal to noise ratios (SNR), resulting in a high dose to the animal. [unreadable] [unreadable] To improve the SNR and to reduce the X-ray dose to the animal, we propose to develop a novel X-ray detector for combined PET/CT system. The detector will be based on a new imaging sensor that unites the properties of an avalanche photodiode (gain) and a conventional CCD (resolution), resulting in a high sensitivity, high resolution imaging system. The system will allow for dramatic improvements in visualization, interpretation, and quantification of PET data using superior anatomical images provided by this CT detector. [unreadable] [unreadable] The availability of a high-sensitivity, high-resolution, large area X-ray detector will allow development of new generation of combined small animal imagers, which provide a powerful tool for studies of such disease as cancer. As such, this detector will be of great interest to the pharmaceutical industry. In addition to the small animal imaging, the detector will have tremendous application potential in areas such as medical imaging, structural biology, nondestructive testing etc. The current market for X-ray detector is estimated to be in hundreds of millions of dollars, a significant fraction of which represents applications where proposed technology would be of great help. [unreadable] [unreadable]